Breathing, and its adjusting during health and disease, is of fundamental physiological importance. In mammals, rhythmic breathing movements and controlled by a central neuronal circuit located within the lower brainstem. Using an in vivo brainstem slice preparation, the properties of neurons located in one part of this neuronal circuit, the dorsal respiratory group (DRG), have been described. The DRG is an important premotor integrating center for rhythmic breathing movements. The long term goal of this research program is to understand the role of neurochemicals in the operation and modulation of the central respiratory circuit. The focus of this proposal will be to extend previous work with brainstem slices by describing the site(s) and mechanism(s) of action of neurochemicals on identified neurons in the region of the DRG. Four drugs will be studied. These are: thyrotropin- releasing hormone, serotonin, substance P, and L-glutamate. All of these neurochemicals have been found within nerve terminals located in the DRG and are known to affect rhythmic breathing movements. In addition, there is ample evidence suggesting that these drugs interact at the cellular level to shape the activity in subgroups of DRG neurons. An in vitro brainstem slice preparation from adult guinea pigs will be used to: 1) define classes (and subclasses) of neurons in the region of the DRG which respond to each drug 2) identify sites within the DRG where multiple drug actions occur 3) determine the mechanism(s) of action for each drug studied The underlying premise of this project is that the neural control of breathing in mammals will not fully be understood until the cellular mechanisms which sculpture the patterns of activity in individual respiratory neurons are adequately described. The use of the in vitro brainstem slice preparation, in combination with sophisticated biophysical techniques such as voltage clamp, offers an exciting opportunity for realizing this goal.